Anyone who drives a public transit bus. The term originated for Toronto Transit Commission bus drivers, though it can be applied to anyone who gets paid 24 dollars an hour + benefits to drive in a circle all day.

The term comes from the fact that their job is so easy a trained monkey could do it.

Bus Driver: "Are you going to pay the full fare?"

You: "Are you going to be here when your schedule says you're supposed to next time, you overpaid lazy piece of shit Bus Monkey?"

A small, dwarf-like person (much like an adult version of an ankle biter) who may or may not be rather obese. When there are no vacant seats on the bus this person has to cling to something, yet, is barely tall enough to reach a bar to hold onto. As the bus turns and thrashes violently, they slide around, using their arms to grab bars, people, or anything to remain standing... just like a monkey would use tree branches.

They are usually quite hairy.

*Short, chubby and hairy man gets onto bus. Bus takes off and he begins to slide about and swing his arms...*